PROJECT SUMMARY Physical activity is critical for blood glucose management and overall health in patients with type 1 diabetes (T1D). Unfortunately, most adolescents with T1D do not meet the recommended ?60minutes of daily moderate-to-vigorous physical activity (MVPA). One reason adolescents with T1D may avoid MVPA is because they fear experiencing uncomfortable glycemic variability. Laboratory studies show a greater occurrence of post- exercise hypoglycemia in adolescents with T1D. However, self-reported activity from field studies show that adolescents? typical MVPA patterns do not mirror laboratory protocols, so it is actually unknown if adolescents experience hypoglycemia (which could present a barrier to MVPA) after their typical MVPA routine. Moreover, only a few studies (all self-report) have measured sedentary behavior (SB). Thus, there are two identifiable gaps in the literature 1- the true impact of MVPA and SB on glycemic variability in adolescents in their natural environment using objective versus subjective measures and 2- identified facilitators or barriers to MVPA according to adolescents with T1D. Collecting the data to fill both of these gaps is the next step to enabling development of targeted behavioral interventions to increase MVPA in adolescents with T1D. The two main objectives of the proposed project are: 1- to examine the impact of MVPA and SB on glycemic variability in adolescents with T1D in their natural environments using state-of-the-art measurement tools (e.g.,accelerometry and continuous glucose monitors [CGM]) as a continuous objective assessment of their typical activity and glucose and 2- to assess adolescents? perceptions of the facilitators and barriers influencing their daily MVPA and SB through mixed methodology. The Applicant will recruit 35 parent and youth (12-17 years) dyads to complete the study. The proposed project is significant because adolescence is a period of steady declines in glucose control and MVPA in T1D and this project will provide a first step towards making it possible to develop interventions targeting MVPA in order to improve glycemic control in adolescents. The project will improve on past methodological weaknesses and may be considered innovative because it will: account for SB, examine all glycemic variability versus just hypoglycemia, provide high density data via CGM and accelerometry, and use a mixed methods approach to assess adolescents? perceptions of the factors influencing their daily MVPA/SB. The project will include career development activities to prepare the Applicant for an independent clinical research career in pediatric diabetes. These will focus on training in diabetes management/physiology, diabetes technologies, exercise assessment, clinical research and trial design, advanced multilevel modeling, scientific writing, and the ethical conduct of research. The project goals will be accomplished with the support of a transdisciplinary and highly experienced team of Sponsors. It is expected that the research project, coupled with the acquisition of new training experiences and interactions with Sponsors and supporting faculty, will advance the Applicant?s training to become an independent researcher.